This invention relates to multi-element sensor or radiator arrays for the reception or emission of propagating waveborne energy for the purpose of determining or communicating the direction of propagation of the wavefront, or for providing a navigation-aid beacon, or for determining the location of a distant emitter or of the sensing or receiving apparatus. In particular, it relates to arrangements in which the total phasor difference between the outputs of pairs of elements, rather than just the phase difference, is taken, its amplitude is detected, and the result is used to determine the direction information of interest. It also relates to arrangements in which both said amplitude and said phase difference are used.
Prior art arrangements of multiple sensor elements are well-known in which the direction information is obtained by first taking the phasor differences between the outputs of individual sensor elements. Such arrangements of four or more elements are as a matter of course around the perimeter of a circle of diameter that at most is substantially equivalent to one wavelength of the subject wave, and by deliberate and calculated design is intended not to exceed this maximum allowable extent. Such arrays are commonly referred to in the art as Adcock antennas, Adcock arrays or structures, or simply Adcocks. In such arrays, the outputs of diametrically separated antenna pairs are differentially combined, although other pairings have been suggested. This method of combining the outputs of two separated antennas is known to yield a phasor-difference signal whose amplitude carries the trigonometric sine of the phase-shift difference between said two outputs of separated antennas, and whose phase is free of any dependence on the direction of propagation of the signal wavefront. For spacing of up to no more than just one wavelength between two antennas whose outputs are differentially combined, the azimuth and elevation angles of the Poynting vector of the propagating wavefront can be calculated from the detected phasor-difference amplitudes of two or more pairs of antenna elements arranged around a circle. However, it is universally presumed in the prior art of Adcock arrays that an uncorrectable error due to spacing between antenna pairs results and renders the array outputs unusable for direction finding for signal frequencies at which the diameter of an Adcock array exceeds a wavelength. This limitation of the diameter to a wavelength or less condemns Adcocks to being small-aperture sensors, and to a severe limitation on achievable direction-finding instrument precision and system performance accuracy that are characteristic of small apertures.
It is therefore an object of this invention to avoid the limitations of a small aperture on instrumental precision and performance accuracy by arranging the discrete sensor or radiator elements around a circle with a diameter well in excess of one wavelength. It is well known that the greater the aperture (or the largest linear dimension, such as the length of a linear array, the diameter of a circular array, the major axis of an elliptical array, and so forth) of an array, the higher the precision and accuracy achievable in the use of the array to determine the direction of propagation of a wavefront.
In certain situations, directions of approach or departure are not all of equal likelihood or importance, certain sectors being favored over others for one reason or another. In certain other situations, mounting platform or space limitations allow greater extensions of the element separations in certain directions than in others, which would rule out arrangement of the discrete elements around circles of diameters equal to the longest available dimension.
It is therefore another object of this invention to determine the direction of propagation of a traveling wavefront from the phasor differences of the outputs of more than one pair of sensor or radiator elements that are arranged along non-circular patterns or rows that conform to specified directional preferences or to available space that lacks circular symmetry.
It is yet a further object of this invention when employing circular arrays of elements (i.e., discrete sensors or discrete radiators) to achieve the favoring of a particular sector of directions by deriving the direction to be determined within said sector from a combination of element-output phasor differences between nondiametrically separated pairs of elements that are symmetrically positioned relative to the axis of said sector, together with the phasor difference of the outputs of the pair of elements separated by the diameter along said axis.
One additional consideration addressed by the present invention is the fact that the successful determination of the direction of propagation of a signal wave from the amplitude of the phasor difference, or from the phase difference, between the outputs of two separated sensor elements is predicated in the prior art on said signal comprising, in the form it is found when intercepted, a filter-separable pure sinwave. While this condition is satisfied by most common signal types, there has more recently been a growing interest in suppressed-carrier spectrum-spread-carrier signals, such as wideband analog or digital FM (e.g., linear-ramp FM, high-deviation-ratio FM by random noise, frequency hopping spread-spectrum) and other (e.g., direct-sequence phase-reversal modulated) signals of all types that do not present a filter-separable discrete sinusoidal component on arrival.
It is therefore a further object of this invention to provide means for operating on spectrum-spread and suppressed-carrier signal outputs of elements in an array to derive therefrom sinusoidal-phasor differences and sinusoid-pair phase differences that correspond to what would be obtained if such sinusoid were present as a discrete spectral line in said signal on arrival.
These and other objects and features of this invention will become apparent from the claims, and from the following description when read in conjunction with the accompanying drawings.